The present invention generally relates to a flow measurement system, with certain embodiments relating to measurement of blood flow characteristics within the heart and large blood vessels using the system for the purpose of controlling electrotherapy.
Physiologic cardiac pacing is very important on a temporary as well as permanent basis. Temporary pacing is usually applied either after cardiac surgery or during myocardial infarction because of the transient conduction disturbance or arrhythmia. Patients at rest have significantly improved cardiac output when ventricular contraction is synchronous with atrial filling of ventricles. This provides for faster recovery after surgery or from myocardial infarction. Furthermore, some arrhythmias like supraventricular tachycardias and extrasystolies may be prevented by means of physiologic pacing. While temporary pacing can be effectively used to aid certain patients as described above, permanent pacing is often necessary for patients having chronic conduction and rhythm disturbance.
As is known, there are two basic modes of physiologic cardiac pacing: sequential and synchronous. For example, sequential atrio-ventricular pacing can be used to restore normal atrio-ventricular relationships. In this mode, an atrium and a ventricle are paced by twin stimuli separated by an appropriate physiologic interval. However, the heart rate is controlled by the pacemaker program and does not vary according to the physiological needs. In contrast, synchronous cardiac pacing approximates most closely to normal cardiac rhythm. The spontaneous atrial electrogram (P-wave) is sensed by an electrode usually in contact with the atrial endocardium. This is used to trigger the ventricle after an appropriate preset delay. Because the atrial rhythm is paced by a patient's natural pacemaker sinus-atrial node, the frequency varies naturally according to the body workload. Therefore, the P-wave synchronous ventricular cardiac pacing can be considered closest to physiologic rate-responsive pacing.
In recent years, cardiac electrotherapy systems have been designed with flow measurement capability. Such functionality allows for measuring a characteristic of blood flow through a specific region of the heart. In some systems, the characteristic may involve blood flow velocity and can be provided through use of a Doppler ultrasonic transducer, which is mounted on a cardiac pacing lead in spaced relation to a pacing electrode at a distal end of the lead. In some cases, when the pacing lead is inserted in the heart, the pacing electrode is placed at an apex of the right ventricle while the Doppler transducer is positioned at or near the tricuspid valve. The flow velocity transducer is generally formed as an annular piezo body having associated electrodes, and is used to measure the flow velocity by means of ultrasound. An ultrasonic lens can be used to direct ultrasonic rays from the transducer and an ultrasonic wave inhibitor structure can be used to prevent transmission of ultrasonic waves in an undesired direction.
In other systems, flow velocity measurement may be provided using non-Doppler means. In such cases, at least two electrodes can be mounted on a lead, with two of the electrodes each being formed of different biocompatible materials. One of these electrodes is formed as a polarizable electrode and is disposed in a detecting position (e.g., at or near the tricuspid valve) and another of these electrodes is located on the lead at an axially spaced distance from the polarizable electrode. In use, detection of over-voltage (caused by variation of ion distribution at the electrode-electrolyte interface) allows for a blood flow velocity signal to be generated.
As described above, blood flow within the heart has been conventionally monitored via a flow measurement sensor on the lead. Signals transmitted from the sensor to a cardiac medical device are in turn used to generate a flow waveform, e.g., via use of a controller within the device. The generated waveforms are often used to show velocity of the blood flowing through a region of the heart, e.g., through the tricuspid valve. In such cases, the controller within the device is responsive to the measured flow velocity, wherefrom the controller can detect heart irregularities and correspondingly control electrical pacing signals to the heart. For example, the flow waveform can be used for synchronization and control of ventricular cardiac pacing. As such, the early rapid diastolic filling wave (E-wave) as well as the late atrial diastolic filling wave (A-wave) can be measured from the generated flow waveforms. Ventricular pacing can then be synchronized with the A-wave. With such cardiac electrotherapy systems, improved and more reliable monitoring of cardiac activity has been achieved, resulting in improved pacing results.
In some cases, the above-described cardiac electrotherapy systems can involve pacemakers which, in normal atrial rhythm, act in a synchronous mode (VDD) and maintain atrio-ventricular synchronism, while only requiring implantation of a single lead. Blood flow velocity measurements can be used in providing rate responsive ventricular pacing and reliable means for atrial fibrillation detection. In addition, continuous monitoring of the right ventricular filling dynamics can be provided in order to estimate the ventricular muscle performance and/or to automatically reprogram the maximum tracking rate in such a way as to prevent angina pectoris and high-rate induced myocardial ischemia. Thus, the above-described cardiac electrotherapy systems, via their flow measurement functionality, can be used to detect a wide variety of cardiac deficiencies, each of which may signify a differing arrhythmic event. For example, the systems can be configured to further identify single premature ventricular contractions, discriminate between sinus tachycardia and pathologic tachycardia, confirm ventricular capture, detect right ventricular failure, etc.
To date, the above systems have generally been limited to acute rather than chronic applications. One reason for this involves tissue which, over time, is found to surround the leads of an implantable medical device. As is known, such fibrous tissue naturally grows or accumulates on the implanted portion of the leads and their corresponding connector assemblies. In turn, body fluid (e.g., water) stemming from such tissue (as well as from blood surrounding the leads) is often found to penetrate the insulative jackets on the leads, resulting in a change in impedance across the lead and connectors. This impedance change can be found to have an adverse effect on the transmittance of the signals along the lead as well as on the signal processing circuitry within the cardiac device. However, the presence of such tissue and/or blood around the leads (and their resulting adverse effect on the leads) is not as prevalent in acute applications because the period of implantation is often short (e.g., ranging from weeks to months) compared to chronic applications in which the period of implantation is much longer (e.g., ranging in years). Therefore, to date, the above limitations have mainly been identified when using the above systems in chronic applications.
What are needed are apparatus and systematic methods to overcome the above limitations so as to enable the above-described cardiac electrotherapy systems to be applicably used in both acute and chronic applications.